1. IRAM Summer School Lecture 3 Françoise COMBES
H2 in External Galaxies
IRAM Summer School Lecture 3
Françoise COMBES

2. The molecular component is essential for the star formation and dynamics
Morphological type
CO in dwarfs
CO in LSB
Radial distribution, spiral structure
CO in bars
Fueling of nuclei
CO in polar rings
CO in E-galaxies, CO in shells
CO in tidal dwarfs

3. H2 content and morphological types
Several surveys of CO in galaxies, Young & Knezek (1989)
more than 300 galaxies in the FCRAO survey
Review by Young & Scoville (1991, ARAA)
The H2 mass is comparable in average to the HI mass in spiral
galaxies
But this could be due to the IRAS-selection for many of them
More recent survey by Casoli et al (1998)
M(H2)/M(HI) in average = 0.2
Varies with morphological type, by a factor ~ 10

4. From Young & Scoville 91

5. H2 content, normalised by
surface or dynamical mass
From Casoli et al (1998)

6. H2 to HI mass ratios
versus type

7. When taken into account the
mass of galaxies
and not only types
Mass is related to metallicity
Z ~ M1/2

8. Tamura et al (2001)
dwarf spheroidals
line = model from
Yoshii & Arimoto 87
Winds and SN ejections in
small potentials
Zaritsky (1993)
dE, Irr, and giant spiral galaxies
abundances measured at 0.4r0

9. For galaxies of high masses, there is no trend of decreasing H2
with type
The dependence on type could be entirely due to metallicity
The conversion factor X can vary linearly (or more) with Z
Dust depleted by 20 ==> only 10% less H2 but 95% less CO
(Maloney & Black 1988)
Environment?
There is no CO deficiency in galaxy clusters, as there is HI
(cf Virgo cluster, Kenney & Young 88)
According to the FIR bias, M(H2)/M(HI) can vary from 0.2 to 1

10. CO in Dwarf Galaxies
Difficult to detect, because of sizes, and low Z (Arnault et al 88)
dE easier to detect than dIrr (Sage et al 1992)
cf the dE Haro2 (Bravo-Alfaro et al 2003)
The more recent observations by Barone et al (98, 00), Gondhalekar
et al (98), Taylor et al (98) confirm a steep dependence on Z
Well know examples: LMC, SMC
X-factor multiplied by 10 (Rubio et al 1993)
even higher below 1/10 of solar

11. Taylor et al 98

12. Only detections are on the right
O/H is the main factor (below 7.9, galaxies are undetectable)
But other factors, too; like the SFR (UV)
Barone et al (2000)

13. Low Surface Brightness LSB
Sometimes large radii, always large gas fraction
(up to fg=95% LSB dwarfs Shombert et al 2001)
and dark matter dominated ==> unevolved objects
Same total gas content as HSB (McGaugh & de Blok 97)
Low surface density of HI, too, although large sizes
Un-compact
Resemble the outer parts of normal HSB galaxies
No CO? (de Blok & van der Hulst 1998), but H2?
Some weak bursts of SF, traveling over the galaxy

14. Scaling properties SDSS - Kauffmann et al 2002b 120 000 galaxies
Different behaviors
for LSB and HSB
SFE lower in
LSB/dwarfs

15. Bothun et al 97
LSB are a reservoir
of baryons
Unevolved, since less
gravitationnally
Unstable
S < Scrit
Poor environments?
High Spin?

16. LSB dwarfs
low masses, small sizes
Masses surface density radius
Schombert et al. 2001

17. Gas fraction vs SB
in LSB dwarfs
compared to models
by Boissier & Prantzos 00

18. CO in LSB, de Blok & van der Hulst 98
But, detection by Matthews & Gao (2001) in edge-on LSB galaxies
M(H2)/M(HI) ~

19. UGC 1922: O’Neil & Schinnerer 2003
OVRO, 109 Mo of H2, with 1010 Mo of HI

20. Same Tully-Fisher relation
(for the same V, galaxies twice as large) M ~V2R
M/L increases as surface density decreases
Low efficiency of star formation (Van Zee et al 1997)
Gas Σg below critical
A gas rich galaxy is stable only at very low Σg
(cf Malin 1, Impey & Bothun 1989)
Galaxy interaction, by driving a high amount of gas
==> trigger star formation
LSB have no companions (Zaritsky & Lorrimer 1993)

21. Tully-Fisher relation
for gaseous galaxies
works much better in
adding gas mass
Relation Mbaryons
with Rotational V
Mb ~ Vc4
McGaugh et al (2000) => Baryonic TF

22. Radial Distribution in Spirals
HI versus H2
The H2 is restricted to the optical disk
while the HI extends 2-4 x optical radius
HI hole or depression in the centers, sometimes compensated by H2 CO HI HI CO

23. Often exponential disks
similar to optical
Radial distribution in
NGC 6946
The HI is the only component
not following star formation

24. Bima SONG (Regan et al 2001)

25. Bima-SONG, radial
distributions

26. Spiral Structure
The H2 component participates even better than the HI and
stellar component to the density waves
due to its low velocity dispersion
Larger contrast than other components
streaming motions, due to the spiral density wave
excitation different in arms? Density
Star formation, heating?
Formation of GMC in arms
Formation of H2? Chemical time-scale 105 yrs
HI is formed out of photo-dissociation of H2
CO exist also in the interarms

27. M51 spiral
+ nuclear ring
Tilanus & Allen
1991
Pearls on string
GMC complexes
More recent map
From Aalto,
Hüttemister et al

28. Full map of M31
with IRAM 30m
CO and dust coinciding
M(H2) < M(HI)
Mvirial > M(CO)
Müller & Guélin 2003
On the Fly map of M31
at IRAM 30m, Neininger et
al (1998)

29. M31 OTF Nieten et al 2000
Müller & Guélin 2003

30. H2 in Barred Galaxies
H2 is particularly useful to map the bar and rings
since HI is in general depleted in central regions
Hα is often obscured
Barred galaxies have more CO emission, and the H2 gas is more
concentrated (Sakamoto et al 1999)
This confirms dynamical theories of transport of the gas by bars
More than half of the gas in the central kpc comes from outside
and is too high (and recent) to have been consumed through SF
Rate of Mo/yr
No relation, however, to the AGN activity

31. Sakamoto et al (99)
CO Survey of barred
galaxies
All kinds of morphologies
Rings, bars,
spiral structure
Twin Peaks
(leading offset dust lanes)

32. Barred galaxies have more concentrated CO emission
and more gas in the central parts

33. Reynaud & Downes
1997
CO trace the
dust lanes
NGC 1530

34. In barred galaxies, star formation is influenced by the gas flow
the resonances, the accumulation in rings
Offset of 320 pc in average between Hα arms and CO arms (Sheth 01)
The gas flows can inhibit star formation, when too fast
(Reynaud & Downes, NGC 1530, 1999)
Favors SF when accumulation in rings, nuclear bars
Two patterns are sometimes required by observations of morphology
and dynamics (cf M100, method of gas response in the potential
derived from the NIR images)
Counter-rotating gas (NGC 3593), or gas outside the plane in galaxy
interactions

35. Simulations of 2 bars in M100
(Garcia-Burillo et al 1998)
Ω = 23 and 160km/s
1 pattern
2 patterns

36. Molecular gas in counter
rotation with respect to
the stellar component
Simulations explain the
m=1 leading arm
Garcia-Burillo et al 00

37. NUGA
LINER
LINER/H
SEYF. 1
SEYF. 1
LINER
SEYF. 2
LINER
SEYF. 2

38. NUGA reveals m=1 instabilities
The new 0.5’’-0.7’’ maps reveal m=1 perturbations appearing as one-arm spirals and lopsided disks in several galaxies. Perturbations appear at various scales, from several tens to several hundreds of pc.
LINER
LINER
NGC4826
NGC1961
Seyf. 1
NGC4579 
150pc
AGN
AGN
AGN  
500pc 
20pc 
0’’.7 (=180pc) CO(2-1) map of NGC1961 shows strong one arm spiral response
0’’.6 (=12pc) CO(2-1) map of NGC4826 shows molecular gas distribution strongly lopsided near AGN.
0’’.6 (=48pc) CO(2-1) map of NGC4579 shows m=1 instability in distribution+kinematics 

39. NGC4631/56 interacting
HI Rand & van der Hulst 93
dust IRAM Neininger 00
Molecular gas out of the planes
also NGC4438 in Virgo
(Combes et al 1988)

40. Polar Ring Galaxies (PRG)
PRG are composed of an early-type host
surrounded by a gas+stars perpendicular ring
The polar ring is akin to late-type galaxies
large amount of HI, young stars, blue colors
Unique opportunity to check the shape of
dark matter halo
But how to relate DM of PRG to DM of
spiral progenitors?
Formation scenarios

41. Molecular gas in Polar Rings
PRG are due either to accreted gas from a companion
or are formed in a merger of two galaxies with orthogonal disk
orientations (Bekki 1998, Bournaud & Combes 2003)
The polar ring is due to gas resettling in the polar plane, but
stars are dispersed
The ages of the stars in the polar ring date the event
CO detected in polar rings (Taniguchi et al 90, Combes
et al 92, Watson et al 1994)
can give insight in the event: metallicity formed from the
recent star formation, or the polar galaxy non destroyed?
Used to determine the flattening of the dark matter
(3D potential probed)

42. Formation of Polar Rings
By accretion?
Schweizer et al 83
Reshetnikov et al 97
By collision?
Bekki 97, 98

43. Formation of PRG by collision

44. Formation of PRG by accretion
Bournaud
& Combes
2003

45. Accretion Scenario
Able to form inclined
polar ring
Gas+stars Gas only
NGC 660 host galaxy contains gas
Probably unstable by precession
even if self-gravitating
In merging scenarios, no formation
NGC 660
CO rich

46. Molecular gas in Ellipticals
Most E-galaxies possess accreted gas, already detected in HI
(Knapp et al 1979, van Gorkom et al 1997)
Either the remnant of the merger event at their birth, or
accretion of small gas-rich companions
Dust through IRAS (Knapp et al 1989), et CO molecules
are also detected (Lees et al 1991, Wiklind et al 1995)
M(H2)/M(HI) times lower than in spiral galaxies
more in field ellipticals
Low excitation temperature
Small gas-to-dust ratio (but correlated to low Tdust)
No correlation with the stellar component ==> accretion

47. D> 30Mpc
Comparison between H2 mass obtained
from CO and FIR (dust)
Wiklind et al (1995)
Lines are for g/d = 700
Dash g/d = 50

48. Shells around ellipticals
The merging events giving birth to ellipticals are also forming shells
Stellar shells known from Malin & Carter (1983) unsharp masking
Schweizer (1983) remark that they accompany mergers
Simulations confirm the scenario (Quinn 84, Dupraz & Combes 87)
The stars of the small companion, disrupted in the interaction
phase wrap in the E-galaxy potential
Recently, HI gas detected in shells (Schiminovich, 1994, 95)
Normally, the diffuse gas condenses to the center in the phase-wrap
process
But CO is now also detected in shells (Charmandaris et al 2000)

49. Formation mechanism for shells: Phase wrapping
Period increasing function of radius
Accumulation at apocenter

50. Star shells
in yellow
HI white
contours
CO points
in red
Radio jets
in blue
Charmandaris, Combes, van der Hulst 2000

51. Blue=stars Red=gas

52. CO dragged outside galaxies
Interactions of galaxies, formation of tidal dwarfs
CO detected in these small dwarfs, supposed to be formed in the
interaction
Braine et al (2000, 01)
Is the molecular gas dragged with the tidal tail gas and reclump in
the tidal dwarf, or the molecular gas re-formed in the collapse?
Trigger some star formation, but in general insufficient to have
solar metallicity
More likely that the gas and metals come from the main galaxies
Fate of these tidal dwarfs? In general, they are re-accreted
and merge

53. Braine et al 2000, 01

54. Conclusions The CO emission depends on type, relative to HI,
but could be only a metallicity effect
Galaxies can have large gas mass fractions, when they have low
surface brightness, and are therefore stable (LSB)
Unevolved, extended, un-concentrated systems, may contain H2
but have low CO emission
No companion, large spin
CO is a good tracer of density waves, spirals, bars, rings
Radial distribution overall exponential, following the optical
But large departures, contrary to stars

55. Elliptical galaxies contain H2, with lower M(H2)/M(HI)
either due to excitation? Different conversion?
Gas coming from accretion
No correlation with stellar component
CO emission very useful to trace density, star formation
perturbations like warps, polar rings, gas dragged out of the
spiral planes